Many wireless protocols and systems have been used or have been proposed for use in controlling lighting systems. Some of these protocols and systems include IEEE 802.11 Wi-Fi, Bluetooth, X10, Z-Wave, INSTEON, nanoNET and ZigBee. Each of these protocols has its advantages and disadvantages. In one proposed ZigBee-based lighting control system, a central device referred to as the coordinator communicates wirelessly with multiple endpoint devices in a star topology. Each endpoint device is a Reduced Function Device (RFD) that is embedded in a lighting fixture. The central coordinator, on the other hand, is a Full Function Device (FFD) that can be made to turn on and to turn off the lighting fixtures by sending Radio Frequency (RF) communications to the endpoint devices. The coordinator and each of the endpoints realizes a ZigBee protocol stack. The first two layers of the ZigBee stack, the physical layer (PHY) and medium access control layer (MAC), are defined by the IEEE 802.15.4 networking standard. Two higher layers of the ZigBee stack, the network layer (NWK) and the application support sub-layer (APS), are specified by the ZigBee standard. User-defined application device objects (ZDO), along with the APS sub-layer, together constitute the application layer (APL) of the ZigBee stack. Additional information on the ZigBee stack can be obtained from The ZigBee Alliance, 2400 Camino Ramon, Suite 375, San Ramon, Calif. 94583, www.zigbee.org.
An endpoint device of the ZigBee-based lighting system receives a RF communication from the coordinator, decodes a command in the communication, and in response to a command in the communication turns on or turns off the light of the light fixtures. The system is “beacon enabled” in that the coordinator periodically transmits synchronizing frames referred to as beacons. The format of the beacons is defined by IEEE 802.15.4. The beacons are transmitted at widely spaced intervals, and between the times of the beacon transmissions the coordinator can spend much of its time in a low-power sleep mode. The endpoint devices receive the beacons and use the beacons to synchronize themselves to the beacon intervals such that the endpoint devices are in a low-power sleep mode at the same time that the coordinator is in the low-power sleep mode. The endpoints are able to wake-up and to enable their receivers synchronously with respect to the coordinator such that when the beacons are transmitted, the receivers of the endpoints are active. After receipt of a beacon, the endpoints turn off their RF transceivers and put themselves back into the low-power sleep mode of operation. It is the NWK layer of the ZigBee stack that manages this synchronization to beacons. Because the relative amount of time the coordinator and the endpoints are active and communicating is much smaller than the amount of time the coordinator and endpoints are in their sleep modes, overall power consumption of the devices is small.
Although such a ZigBee-based lighting control system has many advantages, it does involve an amount of protocol overhead. Supporting unnecessary aspects of the protocol may result in an undesirably large amount of power consumption. In addition, the beaconing period determines the latency between commands and this latency affects the responsiveness of the system to external stimulus. A low latency system dictates a short beacon periods which increases power consumption. Moreover, realizing a full ZigBee stack in a coordinator often requires more than 20 k bytes of memory. Realizing the ZigBee lighting control system can therefore be undesirably expensive. For an extremely cost sensitive battery-powered lighting control application, a less expensive and lower power system is desired.